Page 42 - Geothermal Energy Systems Exploration, Development, and Utilization

18 1 Reservoir Definition
measurements where temperature gradient or thermal conductivity is not indicated,
thus involving removal of the data. At last, a total of 257 data of high quality, 869 of
medium quality, and 517 of low quality were obtained. Figure 1.10 illustrates the
newly obtained heat flow map together with thermal conductivity and temperature
gradient values.
Figure 1.10 does not present any contours of thermal data, which are simply
gathered according to their range of values. Indeed, as previously indicated,
there are many reasons to explain short-scale variations in temperature gradient
and heat flow data. It is thus somehow dangerous to assign a given thermal
regime in a geological area since even lithological changes may lead to significant
local variations. In addition, one can see that thermal conductivity data appear
surprisingly constant in some areas (Ukraine, Belorussia), revealing that the same
value was probably assumed for tens of boreholes. These maps also indicate that
heat flow values are sometimes estimated with no thermal conductivity data (e.g.,
Spain).

1.2.3
Calculating Extrapolated Temperature at Depth

Temperature measurements in mining or petroleum boreholes were used in the last
decades to construct temperature maps at different depth levels (Haenel et al., 1980;
Hurtig et al., 1992). Bottomhole temperature (BHT) measurements in petroleum
boreholes are not necessarily representative of the equilibrium temperatures, and
some corrections are needed (Goutorbe, Lucazeau, and Bonneville, 2007). However,
because of the lack of information, a statistical method is often used to infer
possible equilibrium temperatures. When temporal history of BHT measurements
is well documented, appropriate corrections for transient disturbances can yield
◦
temperature estimates at a few kilometers depth with uncertainties of ±10 C
(Bont´ e et al., 2010).
In the last decade, the extrapolation of European temperatures to 5 km depth
was performed by petroleum industry using such BHT measurements (which
are unavailable), and where any uncertainty propagates and increases with depth
through a linear extrapolation. This map was reviewed and analyzed by Gen-
ter et al. (2003). Using the deepest equilibrium temperature gradients inferred
from measurements in mining boreholes (present in the IHFC database) and
from recent studies (Fern` andez et al., 1998), the authors performed a critical
analysis of the temperature map presented by Hurtig et al. (1992), which was
then modified by the ‘‘heat mining Economic Interest European Group’’ but
only available as an unpublished map. Results of color validation are shown in
Figure 1.11. It must be emphasized that the objective of this work was simply
to use available thermal data to check the interest of some areas which was
deduced from confidential data. When one color code is not confirmed, a tem-
◦
perature difference greater than 20 C is obtained. When it is partly confirmed,
it means that extrapolations are coherent for only a restricted area. The analysis